Function dynify

fn dynify<T: TyVisitable>(x: T, new_self: Option<Ty>, for_method: bool) -> T

Takes a T valid in the context of a trait ref and transforms it into a T valid in the context of its vtable definition, i.e. no longer mentions the Self type or Self clause. If new_self is Some, we replace any mention of the Self type with it; otherwise we panic if Self is mentioned. If for_method is true, we’re handling a value coming from a AssocFn, which takes the Self clause as its first clause parameter. Otherwise we’re in trait scope, where the Self clause is represented with TraitRefKind::SelfId.